With advancement in high performance and high-definition of liquid crystal displays, demanded is a polarizing plate protective film exhibiting durability properties against a severe environment concerning a long-term storage property of a polarizer at high-temperature and humidity, dimensional stability of a polarizing plate, and a curl property of a polarizing plate as a polarizing plate protective film used for the polarizing plate. It is particularly demanded that a long-term storage property of the polarizer at high-temperature and humidity is further improved.
Films containing cellulose ester are recently used for polarizing plate protective films employed in commonly known liquid crystal displays. Though cellulose ester films are prepared by a solution-casting film formation method generally in view of formation of flatness, the films are expanded in the thickness direction with volatilization of solvents, resulting in a low refractive index in the thickness direction in comparison to the in-plane refractive index. A polarizing plate protective film having an equivalent refractive index both in the thickness and in-plane directions and further a polarizing plate protective film having a higher refractive index in the in-plane direction than in the thickness direction are demanded in order to acquire a viewing angle of a liquid crystal display, since a display mode a liquid crystal display, other phase difference films, and a phase difference of members have been taken into account.
It is disclosed in Patent Document 1 that a film, in which degradation of a polarizer at high-temperature and humidity is suppressed and a phase difference in the thickness direction is reduced via addition of an ethylenic polymer, can be obtained. However, it is understood that a recently desired durability level of a polarizer at high-temperature and humidity can not be achieved with a mere addition of the ethylenic polymer. It is also understood that the more the ethylenic polymer is provided, the more degradation of the polarizer occurs.